Cellular networks are constantly evolving. For example, fourth generation (4G) Long Term Evolution (LTE) networks have evolved from third generation (3G), and second generation (2G) radio access technology (RAT) systems. The 3rd Generation Partnership Project (3GPP) LTE standard, in its various releases, is an all-Internet Protocol (all-IP) data transport technology that uses packet-switching for both data and voice communications. By contrast, legacy RAT systems (e.g., 3G/2G) employ circuit-switching for voice communications. Because the evolution of these technologies takes time, cellular networks employ a mixture of newer (e.g., 4G) and legacy (e.g., 3G/2G) RAT systems. Such networks can be thought of as “heterogeneous” cellular networks due to the mixture of legacy and non-legacy technologies.
Mobile devices that are 4G LTE-compliant are configured to communicate within these heterogeneous cellular networks by employing radios that can communicate through the legacy RATs. Generally, 4G LTE-compliant mobile devices are configured to prefer attachment to 4G LTE networks, which offer relatively high data-rate throughput as compared to the legacy radio access networks (RANs). In most mobile devices, a choice of which protocol to employ depends primarily on what RATs are available to the mobile device at its present geographic location. Furthermore, in instances where the preferred 4G LTE RAT is unavailable or unusable, legacy RATs, if available, may be used as a fallback protocol, such as by using a circuit-switch fallback (CSFB) mechanism.
However, with the availability of additional spectrum (e.g., the 700 megahertz (MHz) Band), LTE-based RATs are able to attain better link budget and coverage as compared to legacy RATs due to improved propagation characteristics in the new spectrum. For example, given a single cell site (cell tower), a 700 MHz LTE footprint (i.e., LTE radio coverage area) tends to be larger than the legacy 3G/2G footprint for the same cell site. Without a commensurate enlargement of the legacy 3G/2G footprint, this creates “LTE-only” radio coverage areas (sometimes called “LTE-only areas”) where the only available RAT available to the mobile device is the 4G LTE RAT.
When a mobile device is within an LTE-only area, the only way to deliver voice is through a packet-based protocol, such as the voice over LTE (VoLTE) protocol. Accordingly, an LTE-compliant mobile device within an LTE-only area generally indicates that voice service is available on the assumption that VoLTE can be used for voice communications. However, VoLTE may not always be available or usable in LTE-only areas. Consider an example where a mobile device attempts to establish a call over VoLTE in an LTE-only area and a failure occurs that prevents the use of VoLTE for the call, such as a VoLTE outage. In this example, the mobile devices will implement an event-triggered “retry” procedure where the mobile device, upon detecting that the call cannot be established over VoLTE, tries to establish the call on a different, legacy RAT. In this scenario, the mobile device is unaware that there are no fallback RATs available to it, so the mobile device traverses through all of the possible layers, retrying on each, before it ultimately determines that the call cannot be established. A user of such a device may be frustrated by the failed call attempt, and may be further confused by the fact that the mobile device had indicated, before the call attempt, that voice service was available when in fact it was not (i.e., a false voice coverage indication). In emergency situations, a user of the mobile device may be unable to make a phone call to an emergency services telephone number (e.g., 911) in an LTE-only area when VoLTE is turned off.